Skateboards first became widely popular in the early 1960s, during a surfing craze. Surfers often used improvised skateboards comprised of wooden boards attached to roller skate wheels, as a way to practice surfing when suitable waves were unavailable. As a partial replication of surfing, skateboards are partially recreate the sensation of snowboarding.
Skateboards operate by a rider standing on the board and being propelled either by gravity or by the pumping of his or her legs to propel the board forward. Boards are mainly controlled by the distribution of weight. For example, as a rider shifts weight to the right or left side of the board, it causes the board to rotate slightly along the longitudinal axis of the board, which in most skateboards also causes the wheel axles to turn slightly around a “kingpin” axis tilted forward or backward from the vertical axis, which allows the skateboard to turn right or left. In most skateboards, the wheels are associated with a compliant mechanism such as a spring which causes a resistance force whenever the wheel axles deviate from their resting position, which is usually perpendicular to the longitudinal axis of the board, and parallel to the board's left-right axis.
In most skateboards, the wheel axles are part of an assembly called a truck. A truck typically has at least two parts: (1) a fixed base attached to the underside of the deck, and (2) a movable part, called a hanger, which is attached to the base through a bolt (called a kingpin) at an angle to the vertical axis. The chosen angle for the kingpin may dictate the amount of turning (around the vertical or yaw axis) that will result from a given degree of tilt of the deck around the longitudinal or roll axis of the deck. Between the base and the hanger, there is typically a compliant member, such as a spring or elastic material, which creates force when the hanger is rotated around the kingpin some distance from its rest position.
The hangar portion of the truck typically contains two axles, one on either side, for the wheels. In the resting position, these axles typically protrude at right angles to the longitudinal axis of the board. Skateboard wheels are typically roughly cylindrical, usually but not always rounded edges. Some wheels are also torus-shaped. In normal operation, cylindrical wheels are intended to engage with the paved ground surface, and maintain a region of flat contact with that surface, for maximum friction and to prevent slippage. Torus-shaped wheels have less of a flat surface to engage the paved ground in the wheel's fully-horizontal rest-state, but may be tilted more before losing ground friction.
A problem with prior art skateboard designs is that there is a limit to the amount that the deck can roll along the longitudinal axis, before the wheels tilt so far that they lose their normal engagement with the paved surface. If the wheels are roughly cylindrical, and the truck reaches its maximum rotation around the kingpin, the wheel may tilt upon its edge, leaving only a thin edge to engage the paved surface. Regardless of whether the wheel is cylindrical or more torus-shaped, the tilt may be so great that one of the two wheels on each truck leaves the paved surface entirely, leaving a single wheel to engage the paved surface. The loss of friction caused by the reduction in connection between the wheels and the paved surface is usually undesirable, as it may result in slippage and loss of control.
Thus, in normal operation, skateboards are typically limited in the degree of deck roll they can sustain before there is a loss of control and effective steerage. Thus, traditional prior art skateboards cannot very well replicate the steep angles and turns that a surf board would undergo during surfing or snowboarding. The ability to roll the deck at deep angles and cut tight left and right turns with a skateboard is called carving (borrowed terminology from the field of snowboarding), and the ability to carve deeply is widely recognized as a beneficial feature of skateboards.
There have been a few attempts to overcome the traditional skateboard design to allow deep carving similar to what one might experience in surfing or snowboarding. None of these attempts has been entirely successful, however. One strategy is the use of inline wheels. There have been many inline skateboard designs, a recent example of which is described in WO/2007/034436. This design has the disadvantage of lack of stability. The board has only a thin line of wheels along its longitudinal axis, and the rider must balance on those wheels the way they might balance on one foot while riding on inline roller skates.
Another strategy for making a “surfable” skateboard is to provide the board with swiveling wheels, much like the wheels on a shopping cart. An example of this design is shown in U.S. Pat. No. 6,206,389. This design has a number of disadvantages, among the most important being the lack of control, and the loss of a skateboard “feel” caused the inability to steer the board in the way that one might steer a surfboard or a snowboard.
Yet another strategy, described in U.S. Pat. No. 5,553,874, is to provide trucks with large, curved axles spanning an arc from the right to the left side of the board, and fill the axle with multiple wheels, so that the board can roll left and right at a steep angle. Like the other attempts at creating a surf-like or snowboard-like skateboard, this design has a number of disadvantages, including instability, difficulty of control, and the loss of the traditional skateboard action and feel. Another disadvantage is that the wheels are difficult to replace, as replacement requires significant disassembly.
What is needed is a skateboard truck and wheel design that allows for deep “carving” similar to what one might experience on a surfboard or snowboard. including large angle left and right rolls with accompanying sharp turns, in a manner where the board maintains stability and contact with the paved surface, and the board is responsive to the rider's steering in roughly the same way that steering occurs on a surfboard or snowboard.